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MORPHOLOGY AND EVOLUTION
The exterior of the skull shows sexual differences ; and it is usually easy to distinguish the skull of an adult male from that of an adult female. In some cases, however, it is difficult or even impossible to determine the sex of the skull, for the female may assume male characters or vice versa. Sexual differences are seen in the characters of the temporal ridges and the size of the jaws. In the female the temporal ridges pass upwards, backwards and inwards from the external angular processes of the frontal bones. Beaching the middle line they run parallel to one another, or they are simply in contact.
In the male, on the other hand, they fuse, and the combined ridge may rise up into a crest about 10 mm. high (Duckworth). At the posterior extremity the ridge unites in the male with the nuchal crest to form a lambdoid crest. It is, therefore, 'evident that the skull differs from that of the Gibbon in the characters of the ridges.
Norma Frontalis. — The orbits are oval, with the greatest diameter vertical. The supra-orbital crests are thick and strong, particularly on their lateral parts; they do not communicate across the nose as in the Gibbons, but the crests are confluent in the Gorilla and Chimpanzee.
The articulation of the lachrymal and ethmoid bones has been studied by Regnault (380) and others; and it appears that the os planum of the ethmoid is frequently high anteriorly, so that the lachrymo-ethmoidal suture is long. The lachrymal hamulus is vestigial.
The bony walls of the orbit are more complete than in the Gibbons, for the spheno-maxillary fissure is a